(a) Field of the Invention
The present invention relates to an optical pickup apparatus for recording or reproducing optical media such as optical disks, and more particularly, to an optical pickup apparatus that is close to an optical medium to obtain near-field effect for high density recording or reproduction.
(b) Description of the Related Art
Recently, optical media such as optical and magneto-optical discs have been developed to have high-density storage, to be used for computer storage, package media for music and images, and so forth. In order to obtain high-density storage, it is desirable to reduce the light spot size of the optical pickup apparatus. The light spot size is proportional to the wavelength of the light, and inversely proportional to the numerical aperture of an objective lens system. Therefore, many proposals to reduce the wavelength of the light as well as to increase the numerical aperture of the objective lens system have been suggested.
In order to increase the numerical aperture of the objective lens system, a proposal suggested in U.S. Pat. No. 5,125,750 uses a solid immersion lens (SIL) that may come close to a signal recording surface of the optical disc, as shown in FIG. 10. In the pickup apparatus of FIG. 10, a light source 10 of a laser diode generates laser beams that pass though a collimator lens 12 to become collimated beams. The collimated beams pass through a beamsplitter 14 and then enter an objective lens system having a condenser objective lens 16 and a SIL 18. The condenser objective lens 16 condenses the collimated light onto the SIL 18, and the SIL 18 functions to increase the numerical aperture to focus a light spot on a signal recording surface 19. The light then diffracts and reflects from the signal recording surface 19 back to the SIL 18, the condenser objective lens 16, and the beamsplitter 14. The light is reflected from the beamsplitter 14, it passes through the field lens 22, and then enters into a photodetector 24. The photodetector 24 demodulates the beams to reproduce the original signals.
The light is focused, through the condenser objective lens 16 and the SIL 18, into a light spot having a diameter d that is defined as follows:d˜w/(NA*n)=w/NAeffwhere w is a wavelength of the light, NA is a numerical aperture of the condenser objective lens system in air, NAeff is an effective numerical aperture, and n is a refractive index of the SIL. The effective numerical aperture NAeff is large resulting in very small light spot when an optical material having a large refractive index (generally 2.0 or larger) is used for a SIL.
However, since the prior art optical pickup apparatus use a SIL facing the signal recording surface 19 of the optical disc 20, the SIL 18 may collide with the optical disc 20, thereby allowing destruction of the signal recording surface. Further, it is highly possible for the apparatus or the optical disc to suffer chemical or physical deformation due to high heat caused by light spot radiation.
In order to overcome the above drawbacks, an optical pickup apparatus that collects light from a signal recording surface through a substrate of an optical recording medium has been proposed in Japanese Laid-open Publication No. JP8-221790. In the optical pickup apparatus that is as shown in FIG. 11, a SIL 18′ faces toward a substrate 21 of an optical disc 20, rather than a signal recording surface 19. The SIL 18′ has a planar surface facing the substrate 21, and a semi-spherical surface facing the condenser objective lens 16. A center of the semi-sphere lies on the signal recording surface 19 of the optical disc 20. Therefore, light, which has passed through the condenser objective lens 16, is focused on the signal recording surface 19 through the substrate 21 of the optical disc 20 by the SIL 18′.
The optical pickup apparatus shown in FIG. 11, however, has a restriction in that the SIL 18′ has the same refractive index as the substrate with in range of 1.5–1.55. SILs are generally made of highly refractive material having a refractive index of about 2.0, but the SIL 18′ as shown in FIG. 11 has a refractive index of 1.5–1.55, which is less than the desired refractive index, so it is difficult to obtain a high storage density.
The conventional optical pickup apparatus as shown in FIG. 10 or 11 additionally has a condenser objective lens 16 before the SIL 18 or 18′, so that the apparatus becomes larger and more complex. Further, since the apparatus may be used under the condition that the refractive index of the SIL is the same as that of the substrate of the optical disc, it causes large aberrations of optics resulting in a degradation of the apparatus, as well as restriction in refractive material of the SIL.